"It implies that planets can form more easily than we ever imagined, and that our Milky Way is teeming with planetary systems," said Dr Debra Fischer of Sfsu.

The innermost of the three planets circles Upsilon - and only about 8 million kilometres from its surface. Its 'year' would be only 4.6 days. Although the planet is estimated to be about the mass of Jupiter, it must be a completely different world.

Being so close to its parent star, it must be very hot on its starward side. Its other side may always look away from its sun and may be very cold. Huge storms would rack the planet as heat is passed around the world.

Massive planets

The other two planets are thought to be somewhat larger. One would have a mass of about 2 Jupiters and take 242 days to circle Upsilon, in an oval orbit about 129 million kilometres from the star.

The third planet is even more massive, about 4 times the mass of Jupiter. It is even further away at 400 million kilometres. It takes about 4 years to circle the star.

No current theory of planetary formation predicted that so many giant worlds would form around a star.

"I am mystified at how such a system of Jupiter-like planets might have been created," said Professor Marcy.

"This will shake up the theory of planetary formation," adds Robert Noyes, Professor of Astronomy at the Harvard-Smithsonian Center for Astrophysics.

The research data will be published in the Astrophysical Journal in July. News of the discovery has also been posted on Marcy and Butler's Website and that of the Extrasolar Planets Encyclopedia.